This application claims the benefit of U.S. Provisional Application Ser. No. 60/635,628, filed Dec. 13, 2004, which is expressly incorporated by reference herein.
The present invention relates to an in-fiber whitelight interferometer (WLI) based on Long Period Fiber Grating (LPFG). More particularly, the present invention relates to optical fiber sensors for distance/displacement measurement, near-field surface profiling, vibration measurement, temperature measurement, pressure measurement, refractive index measurement, and bio/chemical detection.
Optical fiber sensors have been widely exploited for confocal microscopy, displacement measurement, temperature sensing, moisture monitoring, and bio/chemical detection, due to their compact size, ultra-fast response, and immunity to electromagnetic interference. The illustrated embodiment provides an in-fiber Michelson interferometric sensor based on the coupling effect of an LPFG. The two optical arms of the interferometer are the light waves propagating along a cladding region and a core region of a single mode fiber. The wave that propagates along the fiber core exits the fiber core, travels in a free space, reflects back from an object, and re-enters the fiber core. The wave that travels in the fiber cladding reflects back at a mirror end of the fiber and interferes with the core wave at a LPFG, thus generating a wavelength-modulated reflectance spectrum, from which the phase delay between these two optical waves can be calculated. Because the phase delay is measured by wavelength shift instead of intensity variation, the illustrated LPFG-based whitelight interferometer is more robust and less noise sensitive.
Distance variation and refractive index change of the free space can alter the phase delay between that wave travels along the fiber cladding and the wave that travels in the fiber core. As a result, the concept of the LPFG-based whitelight interferometry can be applied for distance/displacement measurement, near-field surface profile, vibration measurement, temperature measurement, pressure measurement, refractive index measurement, and bio/chemical detection.
Additional features of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the presently perceived best mode of carrying out the invention.